World energy consumption is predicted to increase 54% between 2001 and 2025. Considerable effort is being directed towards the development of sustainable and carbon neutral energy sources to meet future needs.
Biofuels are an attractive alternative to current petroleum-based fuels, as they can be utilized as transportation fuels with little change to current technologies and have significant potential to improve sustainability and reduce greenhouse gas emissions.
Ethanol is a liquid alcohol made up of oxygen, hydrogen and carbon and is obtained by the fermentation of sugar or converted starch contained in corn grains or converted cellulose from other agricultural or agri-forest feedstocks. The fermentation broth is distilled and dehydrated to create a high-octane, water-free alcohol. Ethanol is blended with gasoline to produce a fuel which has environmental advantages when compared to gasoline, and can be used in gasoline-powered vehicles manufactured since the 1980's. Most gasoline-powered vehicles can run on a blend consisting of gasoline and up to 10% ethanol, known as “E-10”.
In North America the feedstock is primarily corn grain, while in Brazil sugar cane is used. However, there are disadvantages to using potential food or feed plants to produce ethanol and the availability of such feedstock is limited by the overall available area of suitable agricultural land.
The term cellulosic ethanol, describes ethanol that is manufactured from lignocellulosic biomass. There are many different sources of lignocellulosic biomass. The sources may be grouped into four main categories: (1) wood residues (including sawmill and paper mill rejects), (2) municipal paper waste, (3) agricultural residues (including corn stover, corn cobs and sugarcane bagasse), and (4) dedicated energy crops (mostly composed of fast growing tall, woody grasses such as switch grass and Miscanthus).
Lignocellulosic biomass is composed of three primary polymers that make up plant cell walls: Cellulose, hemicellulose, and lignin. Cellulose fibers are locked into a rigid structure of hemicellulose and lignin. Lignin and hemicelluloses form chemically linked complexes that bind water soluble hemicelluloses into a three dimensional array, cemented together by lignin. The complexes cover cellulose microfibrils and protect them from enzymatic and chemical degradation. These polymers provide plant cell walls with strength and resistance to degradation. This makes them a challenge to use as substrates for biofuel production.
Production of ethanol from cellulose via fermentation is a complex process that starts with feed preparation and is followed by biochemical conversion and distillation.
Delivery of biomass starts with selective harvesting, transportation, storing and reducing steps. Biochemical conversion of lignocellulosic biomass to ethanol involves four steps: (1) High pressure treatment of raw lignocellulosic biomass to make the complex polymers more accessible to enzymatic breakdown; (2) production and application of special enzyme preparations (cellulases and hemicellulases) that hydrolyze pretreated plant cell-wall polysaccharides to a mixture of simple sugars; (3) fermentation, mediated by bacteria or yeast, to convert these sugars to ethanol; and (4) ethanol distillation and dehydration.
One variable in the composition of biomass that affects the conversion to energy is lignin. There is evidence that lignin inhibits the process of breaking down biomass to sugars for fermentation. Lignin and some soluble lignin derivatives inhibit enzymatic hydrolysis and fermentation processes. Thus, it is desirable to use a lignocellulosic feedstock which is low in lignin. The lignin content of corncobs, (less than 8% by weight) is low, which would make this a good biomass feedstock for the production of ethanol. However the hemicellulose content of corncobs is high, about 30 to 40% of the total dry matter. Moreover, much of the hemicellulose is acetylated which means that breakdown and liquefaction of the hemicellulose leads to the formation of acetic acid. This is a problem, since the acid is a powerful inhibitor of the ethanol fermentation process. It remains in the pretreated biomass and carries through to the hydrolysis and fermentation steps.
Diverse techniques have been explored and described for the pretreatment of size-reduced biomass with the aim of producing a substrate that can be more rapidly and efficiently hydrolyzed to yield mixtures of fermentable sugars.
These approaches have in common the use of conditions and procedures which greatly increase the surface area to which aqueous reactants and enzymes have access. In particular, increasing the percentage of the cellulosic materials that are opened up to attack decreases the time needed to hydrolyze the cellulose polymers to simple sugars. However, pretreatments of lignocellulosic biomass, such as steam explosion, may result in extensive cellulose breakdown and, to a certain extent, to the degradation of hemicellulose. This results in the production of acetic acid and furfural. Some pretreatment methods employ hydrolytic techniques using mineral acids (hemicellulose hydrolysis) and alkalis (lignin removal).
Pretreatments involving mineral acids (including SO2) primarily solubilize the hemicellulose component of the feedstock while the use of organic solvents and alkalis tends to co-solubilize lignin and hemicellulose.
The resulting product streams (called pre-hydrolysates) are usually separated thereafter into liquid and solid (cellulose) phases. If no separation or detoxification is included in the process, a complex mixture of toxic compounds such as acetic acid and furans will be carried forward to the hydrolysis and fermentation steps. The inhibitory compounds significantly reduce enzyme performance, biocatalyst growth, rates of sugar metabolism, and final ethanol titer due to incomplete conversion of the glucose to ethanol.
The mentioned inhibitors are generally removed through a dedicated step to detoxify pretreatment hydrolysates before fermentation. Detoxification requires additional equipment, e.g. solid-liquid separation, storage tanks, and may also require the addition of chemicals such as calcium hydroxide for over liming, acid for neutralization before fermentation and high yeast nutrient loads, hence added process complexity.
This additional process complexity results in increased capital equipment and operating costs. Therefore, it would be desirable to avoid the need to detoxify completely biomass prehydrolysate prior to the enzymatic hydrolysis and fermentation steps.
Fermentation of sugars by yeast (e.g. Saccharomyces cerevisiae) is the most common method for converting sugars released from biomass feedstocks into fuels, such as ethanol. Yeasts are living organisms, unicellular fungi that need carbon, nitrogen, vitamins, and minerals for growth and reproduction. If compared to corn grain mash, lignocellulosic hydrolysates are not nutritionally balanced for yeast and most need to be fortified with additional macronutrients like nitrogen.
Nitrogen is an essential element needed to avoid sluggish and stuck fermentation. Nitrogen deficiency will cause problems in four fundamental ways. The first three are related to each other as follows: (1) protein synthesis is limited; (2) cell count is limited because the proteins are the bricks used to built new cells, and (3) fermentation kinetics are slowed down due to the reduced cell count. The fourth manner in which nitrogen deficiencies can cause sluggish fermentation is through a decrease in the efficiency of the sugar transporters in the yeast cell membrane, causing a significant decrease in fermentation kinetics at the cellulose level.
Yeast accessible nitrogen is composed of two portions, organic or assimilable nitrogen and inorganic nitrogen (ammonia). Advantageous fermentation broths contain a balance of yeast available nitrogen from both assimilable amino nitrogen and inorganic nitrogen. Therefore, the fermentation step typically requires external nutrient supplementation.
Another major barrier in the efficient use of biomass-derived sugars is the lack of industrial grade yeast that can grow and function optimally in challenging, stressful environments created by lignocellulosic biomass pretreatments as discussed above.
During the fermentation of a detoxified biomass hydrolysate, a significant fraction of available sugar may be diverted by the yeast away from ethanol production to glycerol and succinic acid production. Glycerol production in the yeast is linked to acid, ethanol, and temperature induced stress conditions. The synthesis of glycerol occurs in response to osmotic stress and therefore likely has an essential role in cell viability.
Although yeasts with improved properties such as elevated ethanol and temperature tolerances have been genetically engineered, such strains are not yet used widely by the fuel ethanol industry.
All of the above mentioned problems contribute to the elevated capital cost and operating cost of lignocellulosic ethanol production by reducing product yields, and increasing water volumes that must be handled as part of relatively dilute product streams.
Ethanol production from glucose or from grain or corn starch is now a mature industry. Production of fuel ethanol from sugars present in lignocellulosic biomass, however, remains challenging with many opportunities for improvement.
Thus, improving the throughput and reducing the costs associated with ethanol production from lignocellulosic biomass, is critical to the establishment of a viable industry.